How do I automate a key press in AppleScript?

Is it possible to automate the key press of a key (such as "a" or "<") in Mozilla Firefox, Google Chrome or Safari (in this order)? I'd like to automate the key press of "a" for example in Mozilla Firefox for 100 times every x seconds. With x being a value between 1 second and 10 seconds chosen randomly at every key press. What Applescript could make the trick? Also, where can I find a list of all